1. Field of the Invention
The present invention relates generally to a fuel injection system for internal combustion engines such as used in automotive vehicles and, more particularly, to a fuel injection control method and device for controlling the air/fuel mixture introduced into an engine.
2. Prior Art
Fuel injection systems employing airflow meters have been used in various kinds of automotive engines In a typical system of known type, the airflow meter is installed in the air intake system at an upstream position of the throttle valve to detect accurately the flow rate Q of the air induced into the engine. Then the basic fuel injection quantity Tp, corresponding to the fuel injection duration, is such as to provide a fuel quantity corresponding to the induced airflow rate Q. For example, the basic fuel injection quantity Tp which is close to the theoretical (ideal) air/fuel ratio A/F is calculated in the formula of Tp approximately equals Q/N where N is the engine speed. The fuel injector is basically controlled on the basis of Tp.
A high degree of accuracy is required in the measurement of the engine induced airflow rate Q. Accordingly, precise means such as airflow meters of the hot wire type possessing high accuracy response are used.
Sequential electronic fuel injection systems utilize mass flow measurement to determine air charge (Ma). The air charge calculations are completed at the end of an induction event, at the PIP up-edge interrupt, to provide for an average air charge for that event. A required fuel charge (Mf) is then computed using the desired air to fuel ratio (air/fuel). To provide the best combustion, the resulting fuel charge is injected on a closed intake valve. This is especially important at idle and for engines with low swirl and turbulence.
In today's typical sequential electronic fuel injection (SEFI)/mass air meter control system, the following sequence of events take place in the strategy.
1. First, airflow is measured by a meter mounted up-stream of the throttle body. The manipulation of the raw air meter signal is critical in order to provide a true indication of cylinder air charge.
2. Next, cylinder air charge at the port is determined using a physically based manifold filling model which takes into account parameters such as engine displacement, manifold volume and volumetric efficiency.
3. Once the true cylinder air charge is calculated the corresponding desired fuel charge is then computed: EQU FUEL-CHARGE(lbs)=CYLINDER-AIR-CHARGE / AIR-FUEL-RATIO
The determination of the desired AIR-FUEL-RATIO is complex and requires information from additional sensors, such as temperature, throttle position and exhaust gas oxygen (EGO) sensors as well as sophisticated control algorithms including adaptive fuel control.
4. The required fuel injector pulse width to deliver the desired fuel charge is then calculated, taking into consideration the injector flow rate and offset characteristics.
5. Next, the correct injection timing with respect to the intake valve opening is calculated.
6. Finally, software schedules the injector to deliver the correct pulse width at the required timing.
During steady-state operation, the above calculations are straight forward. However, during transient conditions, accurate and timely fuel control is much more difficult to achieve.
A challenge for the fuel delivery system comes under transient conditions when the throttle is either opened or closed rapidly. Under these conditions, airflow into the cylinder changes very quickly from one cylinder induction event to the next. The ability to cope with these rapid changes is not only determined by the control system hardware, but also by the sophistication of the control strategy.
FIG. 1 depicts what happens with a conventional SEFI/Mass Air Control System when the throttle is rapidly opened and closed at a rate of 500 angular degrees per second. Upon a rapid throttle opening using conventional control strategy approaches, the inherent computational delays result in several consecutive induction events having inadequate fuel delivery, leading to misfire and poor combustion. These characteristics are exhibited by a drop in IMEP (Indicated Mean Effective Pressure) to zero, an increase in air/fuel to above 20:1 and an engine speed drop of 100 RPM. This situation results in a perceived hesitation by the driver, a hydrocarbon spike and a thermal shock to the catalyst which could lead to premature deactivation.
U.S. Pat. No. 4,630,206 discloses a fuel injection system based on computed mass air using an airflow meter. The system compensates for the air charge "calculation delay" problem through multiplying the air quantity obtained in the immediately preceding intake stroke by a ratio of the instantaneous intake airflow rate sampled at a referenced timing in the preceding intake stroke and the instantaneous airflow rate at a referenced timing in the present intake stroke.
With reference to FIG. 6 of '206, air charge Q.sub.1 (throttle not cylinder) is obtained by integrating the instantaneous airflow rates q.sub.1 -q.sub.5. Q.sub.1 is used to compute Q.sub.2, the air charge of the next intake stroke, by multiplying Q.sub.1 by the ratio q.sub.6 /q.sub.1 as seen in equation 4 at column 7. Thus, under a mildly accelerating condition as shown in FIG. 6, the fuel valve opening period is slightly greater at t.sub.2 than at t.sub.1.
A different scheme is used to predict fuel amount under high acceleration, as shown in FIG. 8 of '206, in which additional fuel pulses, e.g., t.sub.22, t.sub.23, are supplied to the engine. To determine whether additional fuel pulses are needed, the system first decides whether the engine is under acceleration as indicated by a throttle sensor or other means (see column 11, lines 52-60). If so, additional fuel is injected based on the computed difference between instantaneous airflow rates in the same intake stroke cycle.
The '206 patent does not calculate cylinder air charge based on a manifold filling model. Instead, it teaches computing the ratio between the instantaneous intake airflow rate sampled at a referenced timing in the preceding intake stroke and that sampled at a referenced timing in the present intake stroke.
U.S Pat. No. 4,911,133 is directed to a fuel injection system which estimates the quantity of air within an intake system downstream of a throttle valve using a model of air within the intake pipe. The patent teaches inferring cylinder air charge based on the total air weight of induced air in the intake system.
U.S. Pat. No. 4,721,087 is directed to a fuel control apparatus which estimates cylinder air charge based on the equation: EQU Qe(n)=KQe(n-1)+(1-K)Qa,
where Qe(n) represents cylinder air charge in the present engine cycle, Qe(n-1) is cylinder air charge in the preceding cycle and Qa is air charge from the throttle flow as measured by the airflow sensor.
U.S. Pat. No. 4,721,087 also teaches a fuel control apparatus with an AN detecting means which detects the output of said airflow sensor at a predetermined crank angle of said internal combustion engine thereby to detect a ratio of said output to the number of revolutions of said internal combustion engine. In an AN detecting means an airflow is represented by A and the engine speed by N so that AN is a ratio of air intake quantity to the number of revolutions of the engine.
Applicants' invention includes predicting air charge two cylinder events into the future. With respect to U.S. Pat. No. '087, Applicants' prediction of air charge takes into account the effect of engine load on volumetric efficiency of the engine in a continuous way. That is, the parameter, k, changes over the entire operating range of the engine. In Applicants' invention all calculations are based on airflow, not on throttle position and/or rate of change of throttle position.
U.S. Pat. 4,911,133 teaches calculating the mass of air in an intake system. In contrast, Applicants' invention calculates only the air mass in the currently filling cylinder.
It would be desirable to further improve the calculation of the amount of fuel needed at a given engine operating condition and to alter the timing of fuel injection into the engine. These are some of the problems this invention overcomes.